It is permissible to offer voluntary prayers atop one’s mount when travelling, no matter what direction it is facing
وَحَدَّثَنَاهُ أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا أَبُو خَالِدٍ الأَحْمَرُ، عَنْ عُبَيْدِ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ يُصَلِّي عَلَى رَاحِلَتِهِ حَيْثُ تَوَجَّهَتْ بِهِ .
Ibn 'Umar reported that the Apostle (ﷺ) used to pray on (the back of) his camel in whatever direction it took him.